1. Field of the Invention
The present invention relates to improving a luminance of a light-emitting device that has a light-emitting element.
2. Description of the Related Art
A light-emitting element that is thin in thickness, light in weight, has a high speed response, and uses a low DC driving voltage has been expected to be applied to a next-generation flat panel display. In particular, a light-emitting device that has light-emitting elements arranged in a matrix shape is considered to be superior in having a wide view angle and a high level of visibility, as compared to a conventional liquid crystal display device.
A light-emitting element is said to have an emission mechanism wherein an electron and a hole respectively injected from a pair of electrodes are recombined in the luminescence center of a layer including luminescent material to form an excited molecule when a voltage is applied to the layer including the luminescent material between the pair of electrodes. Energy is released to emit light when the excited molecule moves back toward the ground state. Both a singlet excited state and a triplet excited state are known, and luminescence is said to be possible through either of them.
When light-emitting elements are arranged in a matrix, the driving method can be passive matrix driving (a simple matrix type) or active matrix driving (an active matrix type). When the pixel density is high, an active matrix type in which a switch is provided for each pixel (or each dot) is considered to be advantageous since low-voltage driving is possible.
One problem in such a light-emitting device is that light can be extracted insufficiently from the light-emitting element. One may provide a reflector for improving a light-extraction efficiency. It is reported, for example, that a wiring of a TFT is used as a light-reflector. For example, refer to Patent Document 1. It is also reported that a metal film is formed into a portion of a substrate in advance. For example, refer to Patent
Documents 1 and 2.
(Patent Document 1)
Japanese published unexamined application No. 2002-229482
(Patent Document 2)
Japanese published unexamined application No. 2002-352950
However, in the foregoing cases, a structure that has the reflector arranged in a position overlapped with a light-emitting region of a light-emitting layer is used when extracting light from the side where the reflector is arranged. Therefore, when an arrangement density of the reflector is made higher, the luminance is not improved since the light-emitting region is narrowed, while the light-extraction efficiency (light intensity extracted outside/light intensity emitted from the light-emitting region) is improved. Alternatively, when the arrangement density of the reflector is made lower to expand the light-emitting region, the light-extraction efficiency is decreased since light emitted from the light-emitting region cannot be reflected by the reflector so much, with the result that luminance is not improved. The arrangement density of the reflector is a ratio of an area of the reflector provided in a position overlapped with a light-emitting surface of a light-emitting layer to an area of the light-emitting region of the light-emitting layer. This ratio may be considered as that of an area of the reflector provided in a position overlapped with a light-emitting surface of a light-emitting layer to an area of the light-emitting region of the light-emitting layer in a pixel.